Braking system for a lift truck

Abstract

A braking system for a lift truck performs all service braking using truck traction drive motors. Mechanical, spring applied, electrically released brakes are coupled to wheels on opposite sides of the truck with the mechanical brakes applying unequal braking forces to the wheels. The mechanical brakes perform park braking and, in the event an electrical system problem arises, backup braking as well that can be modulated by an operator of the truck regardless of the operating condition of the truck.

1. A lift truck comprising:
 a first wheel located on a first side of said truck; 
 a second wheel located on a second side of said truck opposite from said first side; 
 at least one drive motor for driving said lift truck; 
 at least one traction motor controller connected for controlling said at least one drive motor for both moving said truck and for braking said truck, said traction motor controller controlling said at least one drive motor to provide all conventional service braking for said truck; 
 a first brake associated with said first wheel; 
 a second brake associated with said second wheel; 
 wherein said first brake applies a first braking force to provide backup braking, said first brake being operable alone; 
 said second brake applies a second braking force to provide backup braking, said first braking force and said second braking force being unequal to one another; and 
 a brake control for controlling said first and second brakes in the event of a failure resulting in loss of at least one signal for controlling said first and second brakes. 
 
   
   
     2. A lift truck as claimed in  claim 1  wherein said first and second braking forces are based on vehicle geometry to reduce truck skewing. 
   
   
     3. A lift truck as claimed in  claim 1  wherein said first braking force applied to said first side of said truck is equal to X % of a total braking force applied by said first and second brakes and said second braking force applied to said second side of said truck is equal to Y% of said total braking force so that X %+Y %=100% and X %≠Y %. 
   
   
     4. A lift truck as claimed in  claim 1  wherein a total combined braking force applied by said first and second brakes is fractionally divided so that the braking force applied to one side of the truck is equal to a fraction X of said total combined braking force and the braking force applied to the other side of the truck is equal to a fraction Y of said total combined braking force so that X+Y=1 and X≠Y. 
   
   
     5. A lift truck as claimed in  claim 1  wherein said first braking force applied to said first side of said truck is equal to ⅓ of a total braking force applied by said first and second brakes and said second braking force applied to said second side of said truck is equal to ⅔ of said total braking force, said first brake being applied alone or said first and second brakes both being applied so that the most side-to-side braking force difference that can exist during backup braking is ⅓ of said total braking force. 
   
   
     6. A lift truck as claimed in  claim 1  wherein:
 said first brake comprises a first mechanical brake coupled to said first wheel; 
 said second brake comprises a second mechanical brake coupled to said second wheel; and 
 said brake control comprises a brake logic control module for controlling said first and second mechanical brakes. 
 
   
   
     7. A lift truck as claimed in  claim 6  wherein said at least one drive motor for driving said lift truck comprises:
 a first drive motor for driving said first wheel; and 
 a second drive motor for driving said second wheel; 
 wherein said at least one traction motor controller is connected for controlling said first and second drive motors for both moving said truck and for braking said truck, said at least one traction motor controller controlling said first and second drive motors to provide all conventional service braking for said truck. 
 
   
   
     8. A lift truck as claimed in  claim 6  wherein said brake logic control module operates said first mechanical brake alone upon loss of at least one signal for controlling said first and second brakes. 
   
   
     9. A lift truck as claimed in  claim 1  further comprising a mast assembly along which forks are raised and lowered, and a mast height sensor, said at least one signal for controlling said first and second brakes comprising first and second brake release signals, said brake control releasing said first and second brakes in response to said first and second brake release signals, respectively, said brake control operating said first brake alone if either one of said first and second brake release signals is not received and said mast assembly is not above a staging height. 
   
   
     10. A lift truck as claimed in  claim 1  further comprising a mast assembly along which forks are raised and lowered, and a mast height sensor, said at least one signal for controlling said first and second brakes comprising first and second brake release signals, said brake control releasing said first and second brakes in response to said first and second brake release signals, respectively, said brake control operating said first and second brakes upon any request for service braking if either one of said first and second brake release signals is not received and said mast assembly is not above staging height. 
   
   
     11. A lift truck as claimed in  claim 10  wherein said brake control does not operate either of said first and second brakes if either one of said first and second brake release signals is not received, said mast assembly is above staging height and there is no operator request for service braking. 
   
   
     12. A lift truck as claimed in  claim 1  wherein said lift truck further comprises a mast assembly along which forks are raised and lowered, and a mast height sensor, said at least one signal for controlling said first and second brakes comprising first and second brake release signals, said brake control operating both said first and second brakes after expiration of a predetermined time period after either one of said first and second brake release signals is not received and said mast assembly is not above a staging height. 
   
   
     13. A lift truck as claimed in  claim 12  wherein said brake control does not operate either of said first and second brakes if said mast assembly is above staging height. 
   
   
     14. A lift truck as claimed in  claim 1  wherein said lift truck further comprises an operator controlled service brake request device, a mast assembly along which forks are raised and lowered, and a mast height sensor, said at least one signal for controlling said first and second brakes comprising first and second brake release signals, said brake control applying said first and second brakes if truck deceleration is below a given percentage of a requested deceleration rate, said mast assembly is not above a staging height and said service brake request device is activated to a predefined percentage of maximum braking. 
   
   
     15. A lift truck as claimed in  claim 1  wherein said lift truck further comprises an operator controlled accelerator which can be used to command acceleration and service braking, a mast assembly along which forks are raised and lowered, and a mast height sensor, said brake control applying said first and second brakes if truck deceleration is below a given percentage of a requested deceleration rate, said mast assembly is not above a staging height and said accelerator is activated to command maximum service braking. 
   
   
     16. A lift truck as claimed in  claim 1  wherein said lift truck further comprises means for monitoring truck speed, a mast assembly along which forks are raised and lowered, and a mast height sensor, said brake control applying at least said first brake if truck speed is above a requested speed by a predetermined amount, said mast assembly is not above a staging height and a service brake request device is activated to a predefined percentage of maximum service braking. 
   
   
     17. A lift truck as claimed in  claim 11  wherein said brake control operates said first and second brakes if either one of said first and second brake release signals is not received, said mast assembly is above staging height and there is an operator request for service braking. 
   
   
     18. A method for operating a lift truck having first and second wheels located on first and second sides of said truck, respectively, a first brake associated with said first wheel, a second brake associated with said second wheel, an operator controlled service brake request device, a mast assembly along which forks are raised and lowered, and a mast height sensor, said method comprising:
 applying a first backup braking force with said first brake to said first side of said truck; 
 applying a second backup braking force with said second brake to said second side of said truck, said first braking force being unequal to said second braking force and able to be applied alone without said second braking force; 
 generating first and second brake release signals to release said first and second brakes, respectively, when said truck is to be operated; 
 monitoring said first and second brake release signals; and 
 reapplying said first braking force in response to failure to receive either said first or said second brake release signals and said mast assembly being below a staging height. 
 
   
   
     19. A method as claimed in  claim 18  further comprising:
 monitoring said service brake request device; and 
 reapplying said second braking force in response to a request for service braking. 
 
   
   
     20. A method as claimed in  claim 19  further comprising:
 timing a period from initial failure to receive either said first or said second brake release signals; and 
 maintaining application of said first brake and reapplying said second brake upon expiration of a predefined period of time from failure to receive either said first or said second brake release signals. 
 
   
   
     21. A method as claimed in  claim 20  further comprising:
 setting said first braking force to ⅓ of a total braking force that is applied when both said first and second brakes are simultaneously applied; 
 setting said second braking force to ⅔ of said total braking force; and 
 applying said first brake alone or applying both said first and second brakes so that the most side-to-side braking force difference that can exist during backup braking is ⅓ of said total braking force. 
 
   
   
     22. A method as claimed in  claim 18  further comprising:
 applying said first and second brakes if truck deceleration is below a given percentage of a requested deceleration rate, said mast assembly is not above a staging height and said service brake request device is activated to a predefined percentage of maximum braking. 
 
   
   
     23. A method as claimed in  claim 18  wherein said lift truck further comprises an operator controlled accelerator which can be used to command acceleration and service braking, said method further comprising:
 applying said first and second brakes if truck deceleration is below a given percentage of a requested deceleration rate, said mast assembly is not above a staging height and said accelerator is activated to command maximum service braking. 
 
   
   
     24. A method as claimed in  claim 18  wherein said lift truck further comprises means for monitoring truck speed, said method further comprising:
 applying at least said first brake if truck speed is above a requested speed by a predetermined amount, said mast assembly is not above a staging height and said service brake request device is activated to a predefined percentage of maximum braking.